How to convert a callback based code into a Kotlin Coroutine

As you already know, Kotlin Co-routines turns a callback based code block into sequential code. Let me show you an example for the people who doesn’t know already.

val getUserCall = apiService.getUser(15)
getUserCall.enqueue(object: Callback<User> {
    override fun onResponse(call: Call<User>, response: Response<user>) {
        doSomeThing(response.body())
    }

    override fun onFailure(call: Call<User>, t: Throwable) {
        t.printStackTrace()
    }
})

This is how a typical callback based code would look like in Kotlin. But, aren’t we talking about co-routines, why do we still have callbacks?

Ok let me show you the above example using co-routines

launch {
    try {
        val user = apiService.getUser(15)
        doSomeThing(user)
    } catch(t: Throwable) {
        t.printStackTrace()
    }
}

Cool!! but here we assume that apiService.getUser(15) is a function which supports co-rotines. However often that’s not the case, becasue usually we are using a framework or libraries, thats written on/or for Java. In such case, we can’t avoid using callbacks.

In such cases what we can do though is convert these callbacks into Kotlin co-routines. Lemme show you how!!

Let’s take the previous example of getting a user from a api call.

launch {
    try {
        val user = getUser(apiService, 15)
        doSomeThing(user)
    } catch(t: Throwable) {
        t.printStackTrace()
    }
}

suspend fun getUser(apiService: Service, id: Int) = suspendCoroutine<User> { continuation ->
    val getUserCall = apiService.getUser(15)
    getUserCall.enqueue(object: Callback<User> {
		
        override fun onResponse(call: Call<User>, response: Response<User>) {
            continuation.resume(response.body())
        }

        override fun onFailure(call: Call<User>, t: Throwable) {
            continuation.resumeWithException(t)
        }
}

Here we ar using a suspendCoroutine {} to convert a callback into a co-routine. In getUser() first we launch a suspendCoroutine block.

INFO: In order to use suspendCoroutine{} we declare getUser() as a suspend function.

It supplies a continuation as an parameter. We will be using this continuation to inform the coroutine to continue from the point where the function was suspended. We can also pass a result to the continutaion which will be returned when the suspended function returnes.